It is desirable that a seat belt for safely restraining a vehicle occupant be suitably tensioned under a relatively low force when it is withdrawn from a retractor and applied to the occupant so that the occupant is not made uncomfortable and unduly restricted in his or her movements. For this reason, the winding force on the retractor reel is relatively small. Consequently, the belt is wound loosely on the reel. In the event of a collision or rollover that causes the retractor reel to be locked against unwinding rotation, the loose turns of the belt on the reel will be tightened, which allows a functionally significant increase in the length of the portion of the belt by which the occupant is restrained. When the increase due to tightening and tensioning of the turns of the belt on the retractor reel are added to the elongation of the belt due to tension when the occupant is thrown forward, the total elongation of the belt can be undesirably large
The above-described problem is well-known, as is the solution, per se, of providing a belt clamp along the path of the belt between the reel and the occupant, such as next to the retractor on a common frame. The belt clamp operates in a collision by clamping the belt when the belt is subjected to a relatively low force such that very little tightening and elongation of the turns of the belt on the reel have occurred at the time of clamping. Therefore, the elongation of the belt is limited mainly to that resulting from tension in the portion between the belt clamp and an anchor point, and elongation due to tightening of the part of the belt would on the reel is largely eliminated. Examples of known seat belt clamps are described and shown in U.S. Pat. Nos. 4,208,770; 4,323,204; 4,401,281; and 4,422,593, all granted to the inventor of the present invention.
FIG. 3 of the accompanying drawings shows a known belt retractor associated with a belt clamp in a single assembly. The assembly comprises a belt take-up reel 2 rotatably mounted between opposed side walls 1a, 1b of a base portion 1 of a frame bent in the shape of the letter "U". One end of a seat belt 3 is fastened to the take-up reel, and a helical winding spring (not shown) rotates the reel to wind the belt onto the reel. The belt 3 leads from the reel through a gap between a fixed clamping jaw 4 and a movable clamping jaw 6 provided at the free end of a lever member 5 and then is wrapped partway around a corrugated operating roll 8 which is urged toward a release position by a spring 7. A stop tab "S" on the frame side portion 1a stops the lever 5 in the release position The retractor part of the assembly includes a locking mechanism consisting of a pawl 9 and a pendulum (not shown) to move the pawl 9 into engagement with a ratchet 10, which is fixedly mounted on the take-up reel 2, and thereby stop the rotation of the reel. (Refer to, for example, Japanese Utility Model Publication No. 80252/1983.)
With the above-described assembly, the belt 3 is withdrawn from or taken up freely in accordance with the movements of the upper body of the vehicle occupant when the vehicle is stopped or moving normally The emergency locking device operates only in the event of a collision. When the emergency locking device locks the belt in a collision and the tension force in the belt reaches a predetermined threshold level, the operating roll 8 is moved up against the resilient force of the spring 7, so that the corrugated outer circumferential surface 8a thereof engages a corrugated camming surface 6a of the movable clamping jaw 6 and moves the clamping jaw toward the fixed clamping jaw 4, thereby to clamp the belt 3 between the clamping jaws 4, 6 and prevent the further extension of the belt 3.
In the device shown in FIG. 3, it is necessary that the fixed clamping jaw 4, the lever 5 and the associated movable clamping jaw 6, and the corrugated operating roll 8 be attached separately to the frame. Therefore, the steps of assembling the device are restricted greatly and limit the opportunity for improving the efficiency of production to lower the manufacturing cost. Also, the position of the lever 5 which pivotably supports the movable clamping jaw 6 is unstable when the lever 5 is in a non-clamping position. Accordingly, there is the possibility that the lever 5 will collide with the stopper member S due to the vibrations of the vehicle and generate an unwanted noise
In order to prevent the noise problem, the lever 5 or the movable clamping jaw 6 in the conventional clamp is urged toward the release position by a spring in the same manner as the corrugated roll 8. When both the corrugated roll 8 and lever 5 are urged toward the release positions by springs, the number of parts increases accordingly, and the seat belt clamping force decreases, so that the response of the clamp is adversely affected The conventional clamp has room for improvement with respect to this point.